1. Field of the Invention
The present invention relates to a molded lens and a molding tool.
2. Description of Related Art
Optical disc apparatus for performing recording and playback of various information on optical recording media such as CD and DVD have been widely used. An optical disc apparatus includes an optical pickup device for reading information recorded on an optical recording medium. An optical pickup device includes a pickup lens for focusing light from a light source on an optical recording medium.
A pickup lens is generally composed of a convex lens. A pickup lens may be fabricated by molding glass material. A glass pickup lens may be fabricated by injection molding or transfer molding with use of a molding die. For example, the injection molding process supplies glass material into a cavity in a molding die and heats to soften the glass material. It then cools the glass material, thereby producing a pickup lens.
Pickup lenses are produced in large quantities in order to reduce a unit price. A plurality of pickup lenses are manufactured at a time with use of a molding die having a plurality of cavities. For example, a molding die having eight cavities may be used for manufacturing eight pickup lenses at a time. In such a case, it is desired to manufacture pickup lenses having the same quality in each cavity. However, in actuality, manufacturing quality can vary from cavity to cavity. Further, manufacturing quality can also vary from molding die to molding die. In light of this, a technique of providing a marking on a lens is disclosed in Japanese Unexamined Patent Application Publication No. 2002-122711, for example.
A method of manufacturing a glass lens on which a marking is made is described hereinafter with reference to FIGS. 13 and 14. FIG. 13 is a side view showing a manufacturing method of a glass lens 4 on which a marking is made. FIG. 14 is an enlarged side view of a marking portion 3 on the glass lens 4. In this example, the glass lens 4 which has the marking portion 3 that is integrally formed on the top surface of a flange portion 2 is described by way of illustration.
Firstly, glass material is supplied to a cavity in a molding die 10. Then, the glass material and the molding die 10 are heated to soften the glass material. The molding die 10 has a shape that corresponds to a shape of a molded item (the glass lens 4). In this example, a concave portion is formed in a part of an upper mold 10a of the molding die 10 which corresponds to the marking portion 3 as shown in FIG. 13. After that, pressure is applied to the glass material by the upper mold 10a and a lower mold 10b of the molding die 10, and the glass material is then cooled. The glass lens 4 which has the marking portion 3 in a shape that protrudes from the top surface of the flange portion 2 is thereby formed.
However, if the marking portion 3 is formed by the above method, the marking portion 3 and the concave portion of the upper mold 10a come into contact to cause chipping of the marking portion 3. This is because the degree of shrinkage during cooling of the glass material is larger than that of the molding die 10 and, consequently, the glass material shrinks toward the center more significantly than the molding die 10 does. In other words, due to a large difference in the coefficient of thermal expansion between the glass material and the molding die 10, the marking portion 3 can get chipped by contact with the upper mold 10a. If the marking portion 3 of the glass lens 4 and the concave portion of the upper mold 10a of the molding die 10 come into contact as shown in FIG. 14, the marking portion 3 can get chipped. The above drawback can occur in a molded lens which uses lens material having a larger coefficient of thermal expansion than a molding die. Specifically, the above drawback can occur not only in the glass lens 4 but also in a plastic molded lens.
Besides the above method, as other methods of forming a marking portion on the glass lens 4, there are techniques of performing processing afterward, such as writing a marking by a marker pen, changing a deposition mask shape, and performing additional processing. However, those methods require additional work because they cause a trace (marking) error, need cleaning, and so on. In the method using a marker pen, there is the possibility that a marking disappears.